The magnetic behavior of partially electrostatic discharge (ESD) damaged giant magnetoresistive (GMR) heads during thermal and bias current stress is studied. Heads were ESD damaged to a level that would still pass quasistatic specifications and then monitored while at a higher temperature (100 °C or 150 °C) and bias current (5 to 6 mA) for up to 40 h. While controls showed no early changes, significant changes in amplitude, asymmetry, and Barkhausen jump were seen within the first hour of stressing in 10% of the partially ESD damaged heads. Heads that experienced pinned layer reversal or developed Barkhausen jumps were especially prone to large changes in magnetic response during stressing. It is concluded that some partially ESD damaged GMR heads can exhibit spontaneous and large degradation in amplitude and magnetic stability during high thermal and bias current stressing. © 2003 American Institute of Physics.